1. Field of the Invention
The present invention relates to protection circuits for electromagnetic interference (EMI) filters and more particularly to circuits which protect EMI filters from damage due to overvoltage caused by the power transformer and EMI filter components combining to yield a resonant circuit with a center frequency equal to the line frequency of the power distribution system.
2. Description of the Prior Art
In the past, the components of EMI filters were protected from overvoltage by fuses which were calculated to cause an open circuit when excessive currents were drawn through them. The fuses were placed in series with the filter capacitors of the EMI filter. This would prevent high voltage transients from creating excessive currents through the capacitors of the filter.
Using solely a fuse to solve the problem of protecting EMI filters from component failures due to high voltage transients is inadequate in many respects. When the failure of an EMI filter capacitor opened the fuse, all EMI filtering ceased, and the circuit connected to the EMI filter was left with no electromagnetic interference protection. Furthermore, there was usually no indication to the operator that a filter capacitor has failed.